At the end of a St. Patrick Day's gathering, Brian Russell Kaplon and David Andrew Armstrong went into the garage of Armstrong's home in a gated community in Porter Ranch.

Armstrong kept a collection of guns in the garage. What happened next is unclear because Kaplon, an executive at NBCUniversal, and Armstrong, a marketing executive, were the only ones there. But just after midnight March 18, Kaplon suffered a fatal gunshot wound from one of Armstrong's military-style rifles.

At first, Los Angeles Police Department detectives thought the shooting might be an accident. But Thursday they arrested Armstrong, who was charged by prosecutors with one count of murder. Authorities have not provided details of what happened but said they decided to make the arrest after reviewing evidence at the scene.

"Both of the men were in the garage with a rifle when one man shot and killed the other man. There are no other witnesses," LAPD Deputy Chief Kirk Albanese said. "The other man made a statement that was taken into consideration before it was determined to be a homicide."

The Los Angeles County district attorney's office alleged in court papers filed Thursday that Armstrong "personally and intentionally discharged a firearm, a rifle, which caused great bodily injury and death to Brian Kaplon." He faces up to 25 years to life in prison if he is convicted on all charges.

Armstrong's attorney, James Blatt, said the shooting was a tragic accident involving two friends who had been drinking at Armstrong's gathering.

"There will be no motive shown. There was absolutely no animosity or argument before this accidental shooting," Blatt said. "There was no intent ever to harm his best friend. This was an accident based on two friends who were drinking and not following the proper safety precautions in the handling of a rifle."

Blatt said Armstrong called 911 immediately after the shooting and tried to provide medical assistance.

A law enforcement source confirmed that there had been a gathering at Armstrong's home. But someone close to the Kaplon family said Brian Kaplon went there to get a haircut and did not attend any party. This source, who spoke on the condition of anonymity, also said that the men's wives were close friends but that Kaplon and Armstrong were more acquaintances.

Kaplon, who worked in the finance department at NBCUniversal, was described by friends and co-workers as affable and creative. He was married with a child, with twins due in May.

The shooting occurred at Armstrong's home in the 20000 block of Vercelli Way at 12:45 a.m. Kaplon was rushed to the hospital, where he was pronounced dead about 15 minutes later. An LAPD source told The Times that Armstrong was showing Kaplon an AR-15 rifle when a round from the weapon discharged into Kaplon's chest.

Kaplon's father said police had initially told the family the shooting was an accident. But a few days later, detectives returned to say they believe it was homicide. He said Armstrong had several guns in his house, including rifles.

"Because of one of those guns I had to bury my son yesterday," Joe Kaplon said.

Records show that Armstrong was detained by police after the shooting but released. On Thursday, he was arrested.

Blatt said the two families were extremely close.

"Mr. Armstrong, his wife and his entire family are devastated by the terrible loss of his best friend and are hopeful that the truth will come out in the near future that this was a tragic accident," he added. "They were best friends since fifth grade. They were like brothers. Wives were very close. Each family was an integral part of the other family's life."